1. Field of the Invention
The present invention relates to an image reading apparatus capable of reducing the time it takes to perform initial adjustment.
2. Description of the Related Art
Conventionally, a reading apparatus capable of reading a color original in color (hereinafter referred to as a color reading apparatus) generally uses a three-line sensor having color filters of three colors: red (R), green (G), and blue (B).
On the other hand, a reading apparatus that is prepared for reading both a color original and a monochrome original in black and white (hereinafter referred to as a monochrome reading apparatus) uses a line sensor without a color filter, or a line sensor having a color filter with desired spectral sensitivity. In this case, a green-based color filter is used often due to luminosity function characteristics of a human's eye.
In recent years, the colorization of a printing output of a printer and a copier is advancing. Even in the field of scanning (computerization) of an original, a color reading apparatus capable of reading an original in color is spreading. In this case, in order to enable color reading, a three-line sensor is usually adopted as described above.
However, conventionally, the read speed of a monochrome reading apparatus is generally higher than that of a color reading apparatus. Therefore, there is a demand for providing monochrome reading mode capable of reading an original at a speed higher than that of color reading mode in a color reading apparatus.
For example, in the following cases, it is desired that an original is read in monochrome reading mode using a line sensor of one color and one line.
(1) The case where it is previously known that an original is a monochrome original, and a monochrome output is desired instead of a three-color output of RGB as a read signal.
(2) The case where it is desired that a read image is output by a high-speed monochrome printer or a read image is output in a monochrome color at a high speed and at a low cost even by a color printer.
(3) The case where it is desired that the amount of scan data (electronic data) of an original is reduced.
Under such circumstances, a four-line image sensor including a three-line sensor for color (hereinafter referred to as a color sensor) and a line sensor for monochrome (hereinafter referred to as a monochrome sensor) has been developed. Here, the photoelectron accumulation time of the sensor can be shortened by setting the transmittance of the monochrome sensor to be higher than that of the color sensor. That is, the monochrome sensor is capable of reading an original at a higher speed than the color sensor.
For example, Japanese Patent Application Laid-Open No. 2002-111968 discloses an image reading apparatus using a four-line image sensor as a light receiving element. Actually, color reading mode and monochrome reading mode are not selected simultaneously. Then, the sensor itself can be miniaturized by setting a series of paths through which image data in monochrome reading mode (hereinafter referred to as a monochrome mode) flows to be in common with those of color reading mode (hereinafter referred to as a color mode) for the purpose of reducing an analog-digital converter and an image processing portion.
Conventionally, in an image reading apparatus, an output of each image sensor is adjusted by an AD (analog-digital) converter immediately after an application of power so that the output falls within a predetermined input range.
In the image reading apparatus, it is necessary to perform the adjustment of a light intensity of a light source and an analog gain (hereinafter referred to as an initial adjustment) after an application of power. In the case of using a four-line image sensor in which a color mode and a monochrome mode have a common image data path, the sensitivity of a color sensor is different from that of a monochrome sensor, and hence, the initial adjustment needs to be performed respectively for the color mode and the monochrome mode. Therefore, there is a problem that the time it takes to perform the initial adjustment becomes simply twice. On the other hand, in a four-line image sensor having image data paths separately for the color mode and the monochrome mode, the adjustment of both the modes can be completed with one adjustment; however, it is not realistic from the viewpoint of a practical use, a circuit scale, and the like.
Further, in recent years, a power-saving mode such as a standby mode (standby) or sleep mode (sleep) tends to be mounted in preparation for the case where a copier is not operated for a predetermined period of time, from the viewpoint of energy saving of equipment. In an apparatus having a power-saving mode, every time the power-saving mode returns to a normal operation mode, the adjustment of an analog gain and a light intensity of a light source (initial adjustment) are required. Performing initial adjustment twice for the color mode and the monochrome mode every time the apparatus returns from the power-saving mode to the normal operation mode is a substantial problem considering a length of time before the start of reading.